Donald Trump's Paris climate change call will define his presidency

Donald Trump's imminent decision to either quit or remain in the Paris international climate change deal will be his most defining foreign policy call so far.

It will demonstrate the extent of the US President's populist nationalism or pragmatic realism.

Withdrawing the United States would be a massive rebuke to almost every country Trump is trying to work with on a host of foreign policy matters and make his job harder.

US President Donald Trump talks to German Chancellor Angela Merkel at the G7 summit. AP

The leaders of China, Canada, Germany, France, among others, have personally urged President Trump to remain.

Paul Bledsoe, a former White House adviser to President Bill Clinton and now American University environmental and energy policy adjunct professorial lecturer, says: "It's one thing to stick out like a sore thumb in climate denial and his anti-climate policies, it's another thing to stick that thumb in the world's eye."

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No cost to staying in

Quitting would be telling because Trump does not need to take any domestic environmental steps to stay in the historic deal, thanks to an abundance of cheap and clean shale gas and the fact that US emission cut targets are not legally binding.

Yet Trump will feel he needs to appease his hardcore political base and fulfil his campaign pledge to quit.

As a candidate he called climate change a "hoax" invented by China – a key partner of President Barack Obama's in sealing the Paris global accord.

In a veiled swipe at Trump, Germany's conservative leader Angela Merkel, remarked: "The times in which we could rely fully on others, they are somewhat over."

Trump may not care if he becomes an international pariah.

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Without US the deal could unravel

But he and Secretary of State Rex Tillerson should be acutely aware that walking away from the historic climate deal will harm US interests to build support on other important security, economic, trade and diplomacy objectives.

None less so than with China's President Xi Jinping, who Trump is trying to woo to help deal with North Korea's nuclear weapons program and to get a better trade deal from Beijing for the US.

Trump's most pragmatic course of action would be to grumble loudly and stay in the historic Paris accord, albeit with some strings attached, such as a review or possible weakening of US emissions targets.

Without the participation of the US, the world's largest economy and biggest historic carbon emitter, the Paris deal could unravel.

Evolving views

The "remain" camp in Trump's inner circle include his influential son-in-law Jared Kushner, daughter Ivanka and former Goldman Sachs executive turned economic adviser Gary Cohn, as well as Tillerson, a former ExxonMobil chief executive.

After the Pope urged Trump to stay in the accord, Cohn said last week the President's views were "evolving".

Nevertheless, much of the modern Republican Party believe man-made global warming is bunkum, despite elders such as former senior cabinet members from the Reagan and Bush administrations – James Baker, George Shultz and Henry Paulson – imploring Trump to adopt a carbon tax.

Fierce nationalists such as chief strategist Stephen Bannon and Environmental Protection Agency head Scott Pruitt are leading the "leave" campaign.

A nation of cheap gas

As the former attorney-general of Oklahoma, Pruitt successfully sued to stop the enforcement of President Obama's clean power plan and is now working to strike out the anti-coal regulations that were designed to help the US meet its Paris obligations.

The great irony is Trump could stay in the deal, with little or no downside.

The US is on track to meet its 2020 target of a 17 per cent emissions reduction on 2005 levels, thanks largely to price plunges for natural gas and solar, which have made dirty coal less attractive.

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Gas-fired electricity generation exceeded coal-powered generation for the first time in the US last year.

Obama's bolder 26-28 per cent emissions reduction by 2025 will be harder to meet and require new policy steps. Trump is trying to gut some of Obama's climate policies, especially on coal, so the target is in further doubt.

No congressional approval required

Regardless, the US wouldn't cop a penalty for exceeding the voluntary goal.

The Paris deal, unlike the flawed Kyoto pact that President George W Bush rejected, was intentionally not legally binding, partly because a legally enforceable deal would have required US Congress approval.

Nigel Purvis, a former senior US climate change negotiator in the Clinton and Bush administrations, recalls the damaging impact Bush's withdrawal had on the US' international reputation.

"The decision by Bush to reject Kyoto really became a broader symbol of him being unilateral and go-it-alone American power that built up resentment in the international community," Purvis says.

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Trump's best climate contribution could be to fulfil a promise to unleash a liquefied natural gas export boom to help replace coal around the world.

Bledsoe says this would have the added benefit of slightly raising the price of gas in the US and making zero-emissions nuclear power price competitive.

With this unpredictable President, anything is possible.

The bottom line is Trump would risk losing foreign partners and harm the US' broader international objectives by abandoning Paris.

John Kehoe is US Correspondent for The Australian Financial Review in Washington.